This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The McCollum lab studies a signaling pathway in the fission yeast S. pombe, termed the septation initiation network (SIN), which functions at the end of mitosis to trigger initiation of cytokinesis. Most of the components of the SIN pathway have been identified, however their precise molecular interactions and the role phosphorylation plays in their regulation have not been determined. We have been using mass spectrometry to characterize molecular interactions among SIN components and identify sites of regulatory phosphorylation. These studies have focused on the Sid2-Mob1 kinase complex and a positive (Clp1) and negative regulator (Dma1) of the SIN. All of the components of this pathway are conserved in humans and function in cytokinesis and in tumor suppression. These studies should lead to a more comprehensive understanding of the SIN pathway in yeast, which we hope will help in the characterization of tumor suppression pathways in humans.